The discovery and characterization of single-gene obesity syndromes in the mouse has led to dramatic progress in our understanding of the neuroendocrine control of energy homeostasis. Cloning of the obesity locus led to the discovery of the adipostatic hormone, leptin, while characterization of the agouti obesity syndrome led to the finding that the arcuate POMC neurons exert a tonic inhibitory effect on feeding and energy storage. Agouti encodes a peptide normally expressed only in skin that regulates pigmentation by acting as an antagonist of the melanocyte- stimulating hormone receptor (MCl-R). Mice containing certain dominant alleles of the agouti peptide appear to become obese because ectopic expression of the peptide in the brain aberrantly antagonizes the related hypothalamic melanocortin-4 receptor. Intracerebroventricular (icv) administration of MC4-R agonists and antagonists in the mouse were used to test this hypothesis; icv administration of melanocortin agonists inhibited feeding, while an antagonist was stimulatory (Fan et al., 1997). This finding was corroborated by deletion of the MC4-R from the mouse, which recapitulated the unique constellation of phenotypes seen in the agouti obesity syndrome (Huszar et al., 1997), including hyperphagia, hyperinsulinemia, obesity, and increased linea4r growth. Recent studies have identified a second agouti brain, is expressed almost exclusively within the arcuate nucleus of the hypothalamus. Like deletion of the MC4-R or ectopic expression of agouti, ubiquitous over-expression of AGRP causes the agouti obesity syndrome. These data argue strongly that POMC peptide agonists and the AGRP antagonists act in concert on the MC4-R to regulate energy homeostasis just as agouti and alpha-MSH act in concert on the melanocyte to determine pigmentation. As this grant was being completed, two independent cases of familial obesity resulting from deleterious mutations in POMC were reported, demonstrating related pathophysiology in humans as well. However, while pathophysiological disruption of MC4-R signaling causes obesity in these models, little is known regarding the normal hormonal, nutritional, or neural inputs to energy homeostasis that are dependent upon the POMC neurons for their transmission. Furthermore, little is known regarding the mechanisms by which the central melanocortin system regulates energy homeostasis and how information derived from the melanocortin system integrates with other pathways known to be involved in regulation of energy homeostasis. The melanocortin system may well be important in common forms of human obesity since a quantitative trait locus for obesity maps near the POMC gene. In this project we will utilize genetic pharmacological, physiological, and neuroanatomical approaches in the mouse to determine the physiological inputs to the melanocortin system relevant to the regulation of feeding and metabolism, and to characterize the mechanisms by which the central melanocortin system regulates energy homeostasis.